Circulating fluidized bed (CFB) boilers are used in the production of steam for industrial processes and electric power generation; see, for example, U.S. Pat. Nos. 5,799,593, 4,992,085, 4,891,052, 5,343,830, 5,378,253, 5,435,820, and 5,809,940. For an overview of the design and operation of CFB boilers, see Steam: Its Generation and Use, 41st ed., Chapter 17 (2005; The Babcock & Wilcox Company, Barberton, Ohio) and Steam: Its Generation and Use, 42nd ed., Chapter 17 (2015; The Babcock & Wilcox Company, Barberton, Ohio).
In a CFB boiler, upward gas flow carries reacting and non-reacting solids to an outlet at the upper portion of the furnace where the solids are separated from the gas, typically by a staggered array of impact-type particle separators. The gaseous components of the stream navigate around the separator unit, while the entrained solids deflect and return to the furnace bottom. Impact-type particle separators, which generally are not cooled, protect the downstream heating surfaces, such as those of primary and secondary superheaters, from erosion by solid particles.
While such separators can have a variety of configurations, a common version involves so-called U-beams, individual impingement members having U-shaped cross sections.
Because of the extremely high temperatures experienced at a furnace outlet, the materials from which particle separator components, such as U-beams, are made must be sufficiently temperature resistant to provide adequate support and resist damage. Some impact-type particle separators are cooled or supported off a cooled structure; see, for example, U.S. Pat. Nos. 6,322,603, 6,454,824, and 6,500,221.
A representation of a commercially available CFB boiler with impact-type separator is shown in FIGS. 1, 2 and 2A. Furnace 10 has a gas-tight enclosure 11 suitable for operating with a positive pressure in furnace 10 and a flue gas flow path 15. Primary air enters furnace 10 through windbox 80 and distribution grid 90 (also known as a distributor plate), and, downstream thereof, secondary air is injected through upper and lower overfire air headers. Fuel and sorbent are fed to the CFB bed through the lower walls of furnace 10, with ash and spent sorbent being removed through drain pipes in the floor. The primary solids separation system, generally designated 30, includes staggered rows of U-shaped channel members, i.e., primary particle separator U-beams 32 and in-furnace U-beams 34, suspended from the roof or other pressure parts of the unit. Solids collected by U-beams 32, 34 and multi-cyclone dust collector are returned through the rear wall to the lower portion of furnace 10.
Situations where a plant loses power, sometimes referred to as a “black plant” condition, call for procedures and equipment which permit boiler pressure to decay and boiler temperature to cool to stable conditions as quickly as practical, all without allowing the water level to drop below the furnace roof. Typically, a main steam stop valve (MSV) closes to prevent rapid pressure reduction in the steam/water side and dropping of the water level in the boiler. Thermal inertia in the drum, tubes, headers and other boiler components continues to generate steam after the MSV closing, however. Opening of a steam relief valve prevents steam pressure buildup, which can trigger a safety valve opening with a corresponding rapid water level drop in the boiler and can provide cooling of superheater surface subjected to residual heat of the uncooled parts of the boiler components, such as a U-beam solids separator. The opened valve allows steam to bleed through the steam side of the superheater into the atmosphere or to the steam user (when the steam is used for heating), typically in a controlled manner.
Such steam bleed lowers the water level in the boiler circulation system, however. If the water level recedes below the furnace roof, portions of the tubes become uncooled, and the residual heat of the uncooled parts of the solids separator can damage the uncooled tube portions. To prevent such damage by maintaining a safe water level in the boiler, the boiler can be provided with sufficient steam drum capacity and/or an independently powered boiler water pump, also known as a dribble pump. Both of these increase boiler cost, however.
An attempt to ameliorate these additional costs is described in U.S. Pat. No. 8,047,162, where steam bleed is controllably discharged into the boiler furnace. The steam bleed temperature typically is on the order of 400° to 600° C. (750° to 1100° F.) lower than that of the uncooled parts of the solids separator, so introduction of steam into the solids/gas flow path accelerates the cooling of (potentially) uncooled tubes in the vicinity to a safe temperature (˜500° C.). This solution can reduce the amount of the extra steam drum capacity, but it does not usually altogether eliminate the need for additional capacity nor for the independently powered boiler water pump.